It is already known to subject rice to hydro-thermical treatment, i.e, a treatment under the influence of moisture and heat and to market a type of rice treated in such a way as parboiled rice, in which type the cooking time is considerably reduced. Other starchy products can also be subjected to such a hydrothermical treatment for the same purpose.
It has already been suggested to soak the rice in water, to cook it after that and then to remove the grains of rice from the cooking water and to dry them. It is true that for such a process simple and cheap installations are sufficient, but such a method requires a great expenditure of energy, is very time-consuming and can only be used for treatment of rice in batch quantities. Such a process does not lend itself to industrial use, though.
Further, it is also already known to carry out the cooking process in a pressure vessel (GB-A-722 333, GB-A 781 062, GB-A 807 084). It is true that in doing so the cooking process is accelerated, but the machinery required for this is relatively expensive and its operation requires--if only because of legal regulations--an especially trained operating staff. With this known procedure, the expenditure of energy is relatively great as well and furthermore this method allows only a treatment of the rice in batch quantities.
Also it has already been proposed to treat the rice with water and steam simultaneously (US-A 2 592 407, US-A 3 086 867). In this case, the treatment generally consists of a multi-stage process, which requires even greater expenses for equipment and involves a greater expenditure of energy.
From the GB-A 179 206 a method and an apparatus for the production of parboiled rice are known in which the heating of the rice in the cooking process takes place by contacting it with a hot surface. The rice swollen by being soaked in unheated water is carried through shafts whose walls are heated up. In order to bring the rice to boiling temperature in this process, the passageway through the shafts must be either very long, which entails high investment cost and requires considerable space, and/or the cross-section of the shafts must be very small to improve the ratio of quantity of rice per unit of thermal energy, and/or the heat supply must take place with a very high inlet temperature. In the latter case, those grains of rice which are in immediate contact with the shaft wall are in danger of being heated more intensely than the grains being in the middle of the shaft so that no simultaneous treatment can take place. Furthermore, roasting effects and even scorchings of the grains of rice with a corresponding change in their flavour may be brought about.